Eclipse #141

• Break up a pair. Two identical symbols side by side cannot become three. Both cells flanking the pair take the other symbol.
• Fill a gap. A gap between two identical symbols must hold the other symbol - otherwise you would have three in a row.
• Only one way to finish the line. Work out every legal way the line could be completed. Any cell that reads the same in all of them is settled, even if no single simple rule proves it.
• Assume and refute. Suppose a cell is a sun and follow the simple rules. If that leads to an impossible board, the cell must be a moon. You never keep the guess - you only ever rule one out.
• Line is full. Each row and column holds exactly half suns and half moons. Once one symbol has filled its half, every remaining cell in that line is the other symbol.

Every one of these is explained properly in the Eclipse guide.